We too had one of these and I seriously recommend that if you have more than one cat, stay away from these because they're just not powerful enough to keep up with more than one cat.

We have three cats and even with the finest (smallest grains) clumping litter it still wasn't strong enough to keep up with the loads of three cats. What's worse is that it works on a sensor so when a cat leaves the litter the littler sweep engages. The problem with this is that if one cat decides to enter the litter box after another leaves, the sensor gets confused and doesn't know what to do. It will, at times, engage while a cat is in the litter because cats tend to be pretty still while "going"

Hope this helps.